so i guess you need to specify which holiday calendar you want to use
as an argument to the isBizday function. it is not clear to me where
to find these holiday calendars.
the help pages for fCalendar mention a database holidays.db. however,

> data(package="fCalendar")

discloses only the timezones.db data set

as an aside, if you are in vienna, i think you want timeDate
("2005-12-08", zone = "Vienna") rather than what you wrote. this
will ensure your timeDate object actually has the date you specify.
for example,